1. Field of the Invention
The present invention relates to a method of manufacturing multiple-layer molded articles with an outer skin of compact thermoplastic material and a core of foamed thermoplastic material. The method includes initially introducing the material for the skin into a mold cavity in a quantity sufficient for the desired thickness of the skin and subsequently feeding into the mold cavity the material for the core to which an expanding agent has been added.
2. Description of the Prior Art
In a method of the above-described type known from British Patent 1,303,517, initially a certain amount of thermoplastic material for forming the skin is injected into a mold cavity. Subsequently, the thermoplastic material for forming the core to which an expanding agent has been added is introduced in such a quantity that it completely fills that portion of the cavity which has not been filled by the material for the skin. Immediately thereafter, the volume of the mold cavity is enlarged, so that the thermoplastic material which was supplied last can be foamed as a result of the expanding agent contained therein. A portion of the material of the skin penetrates in and fills out the additionally freed volume of the mold cavity.
This known method can only be carried out with the use of so-called breathing tools. Such tools are difficult to manufacture and maintain. This known method has the additional disadvantage that molded articles manufactured according to this method have a relatively high finished rate because, for ensuring a proper molding of the article, the core material which is foamable or to which an expanding agent has been added is required in a relatively large quantity in order to provide a sufficient internal pressure. However, this pressure can only be obtained if excessive foaming of the core material is prevented.
It is also already known to manufacture molded articles only from a plastics material to which an expanding agent has been added. In this method, the plastics material containing the expanding agent is introduced into the mold cavity against a gas pressure which is higher than the pressure which is generated by the expanding agent. Gas is then additionally introduced into the mold cavity in order to uniformly distribute the plastics material along the walls of the entire mold cavity. Finally, the pressure of the gas in the mold cavity is lowered, so that the expanding agent causes foaming of the plastics material towards the inside and, thus, the hollow space previously formed by introducing the gas is completely filled with foamed structure.
The known method described above has the disadvantage that it can only be carried out with very well sealed molding tools. Such molding tools are difficult to manufacture and maintain.
In accordance with another known method of manufacturing molded articles of plastics material, a plastics material without expanding agent is introduced into the mold cavity and is placed into contact with the walls of the cavity by subsequent introduction of a gas. A sudden pressure drop of the gas then causes a partial interconnection of the plastics material which is still molten in the interior into a so-called woven structure. As a result, relatively large hollow spaces are formed in the interior of the molded articles, with webs being irregularly distributed within the hollow spaces.
The formation of the webs within the molded articles is only random. In other words, the reproducibility of the inner structure of the molded articles is extremely low.
Another known method of manufacturing molded articles is the so-called gas injection method. In that method, the molten plastics material is injected without expanding agent into the mold cavity and is subsequently completely placed against the walls of the mold cavity with the aid of a gas. This method results in molded articles which have large hollow spaces.
However, the molded articles manufactured in accordance with this method cannot be subjected to great loads during use, because they easily fail due to the presence of the large hollow spaces.
It is, therefore, the primary object of the present invention to provide a method of manufacturing molded articles with a compact skin layer and a foamed core which, in spite of obtaining significant density reductions in the core portion, ensure a problem-free formation of the skin portion or surface portion and which, moreover, requires a reduced dwell time in the injection mold.